20 Ga. #3 buckshot through .565 constriction
#2
Try it at different ranges, 10 yards, 20 yards, 30 yards, etc, etc, etc.
Thats the only way to see how it will shoot.
JMHO
Good Luck
#3
what is your standard full choke constriction? i would prolly do what jrbsr said but start off at 30 yards. shoot one with that choke then your full choke at a big target. get a pellet count and go from there. in my opinion shells are to expensive so i wouldt shoot 10 or 20